Clay Mineral Characteristics Of Sanshui Basin From Lower Cretaceous To Eocene And Their Response To Tectonic Activities And Climate Changes In The Region

The Sanshui Basin is a continental basin in northern margin of South China Sea(SCS),developed from Cretaceous to Paleogene.It experienced the subduction of proto-South China Sea in Late Mesozoic and the continental rupture in modern South China Sea's initial evolution in Early Cenozoic.The development of Sanshui Basin is associated with the evolution of the South China Sea.Clay mineralogical analysis has unique advantages for regional tectonic-climate reconstruction.However,there are few studies on the characteristic of clay minerals in Sanshui Basin.Based on above,this paper systemically studies the characteristic of clay minerals of sedimentary rocks from Cretaceous to Paleogene in Sanshui Basin,and has a systematic understanding the composition and distribution of clay minerals in the basin.In addition,combined with outcrop observation and other data,this paper identifies the tectonic-climate evolution from Early Cretaceous to Eocene.The main conclusions of this paper are as follows:(1)There are three types of clay mineral assemblage from the Cretaceous to Paleogene in the Sanshui Basin:the first type is illite+chlorite combination;the second type is illite+chorite+kaolinite;the third type is illite+kaolinite+chorite.(2)The composition and content of clay minerals in different strata of basin are different,and the content of clay minerals of same formation in different regions are also different:The relative content of clay minerals of Sanshui Formation in the northern part of basin is obviously different from that in the eastern and central part of the basin.However,there is not much different in the relative content of clay minerals of Dalangshan Dormation and Xinzhuangcun Formation in different regions of the basin.(3)The clay minerals in the basin are mainly from the detrital sources,and are not effected by diagenesis.So they can be used to indicate the change of the climate and source.According to the clay mineral characteristic curve,the tectonic-climate evolution from Early Cretaceous to Eocene can be divided into five stages:the first stage is from sedimentary period of Baizushan Formation to the late sedimentaryperiod of Baihedong Formation.In this stage,the tectonic uplift was slow and the tectonic activity was not strong,and the climate was drought.The second stage is from late sedimentary period of Baihedong Formation to late sedimentary period of Sanshui formation.During this period,there was a strong regional tectonic uplift,and the climate was mainly arid.The third stage is from late sedimentary period of Sanshui Formation to early sedimentary period of Xinzhuangcun Formation.In this stage,the climate was arid and semi-arid,and the tectonic activity weakened,and the regional tectonic uplift slowed down or stopped.The fourth stage is from early sedimentary period of Xinzhuangcun Formation to late sedimentary period of Baoyue Formation.During this period,the characteristic of clay minerals was dominated by climate changes,and climate changed from arid and semi-arid to humid,and finally to arid and semi-arid.The fifth stage is sedimentary period of Huayong Formation.In this stage,the source of the basin are sedimentary rocks,not volcanic rocks,and the shrinkage and closure of the basin caused the gradual increase of chlorite content,the climate changed from arid to semi-arid or humid.(4)The climate of Sanshui Basin in Cretaceous was more drought than that in Paleogene.This is because there was a huge mountain range in northern margin of South China Sea in Cretaceous,which was casued by the subduction of proto-South China Sea,and this mountain range blocked the warm and moist air from interior basin.